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Abstract:

A method for acoustic tomography within a patient may include generating
a focused ultrasonic signal using a transducer is provided; the
ultrasonic signal forming a path within the patient. The method includes
directing the ultrasonic signal on a spot within the patient; scanning
the spot in a predetermined pattern about a volume within the patient;
receiving an ultrasonic echo in the transducer; converting the ultrasonic
echo into a voltage; selecting a frequency band from the voltage;
amplifying the voltage in the selected frequency band with a processing
circuit; and generating an image of the volume within the patient
structure utilizing the amplified voltage. A method for recanalization of
a blood vessel including the above acoustic tomography steps is also
provided.

Claims:

1. A method for acoustic tomography within a patient, the method
comprising: generating a focused ultrasonic signal using a transducer,
the ultrasonic signal forming a path within the patient; directing the
ultrasonic signal on a spot within the patient; scanning the spot in a
predetermined pattern about a volume within the patient; receiving an
ultrasonic echo in the transducer; converting the ultrasonic echo into a
voltage; selecting a frequency band from the voltage; amplifying the
voltage in the selected frequency band with a processing circuit; and
generating an image of the volume within the patient structure utilizing
the amplified voltage.

2. The method of claim 1 wherein the converting the ultrasonic echo into
a voltage includes using an Application Specific Integrated Circuit
(ASIC) proximal to the transducer.

3. The method of claim 2 wherein the ASIC and the transducer are
positioned in a distal end of a catheter.

4. The method of claim 3 wherein the scanning the spot in a predetermined
pattern about a volume comprises rotating an imaging core about a
longitudinal axis inside the catheter.

5. The method of claim 4 wherein the rotating an imaging core comprises
rotating a reflective element about a longitudinal axis of the catheter.

6. The method of claim 1 wherein generating a focused ultrasonic beam
comprises generating a focused beam having a bandwidth that is more than
70% of the center frequency.

8. The method of claim 1 wherein scanning the spot in a predetermined
pattern includes forming a helical pattern.

9. The method of claim 1, wherein the volume within a patient comprises a
wall of a blood vessel; and the providing an image from the echo signal
includes providing a cross section of the blood vessel.

10. The method of claim 10, further comprising classifying a plaque
within the wall of the blood vessel.

11. The method of claim 11 wherein classifying the plaque within the wall
of the blood vessel comprises rendering a characterized tissue component
map.

12. The method of claim 11 wherein rendering the characterized tissue
component map comprises: spectrally analyzing the ultrasonic echo;
distinguishing different plaque components in the characterized tissue
component map; assigning identifying values to the different plaque
components; applying a classification criterion to spatially arranged
data of the characterized tissue map; and rendering a plaque
classification associated with the cross section of the blood vessel.

13. The method of claim 1, wherein the ultrasonic signal has a frequency
in the range from 5 MHz to 135 MHz.

14. The method of claim 1 wherein the generating an image of the volume
within the patient includes reconstructing a three-dimensional (3D) image
using two-dimensional data.

15. The method of claim 1, wherein the generating the ultrasonic signal
comprises directing the ultrasonic signal along a direction substantially
parallel to a longitudinal axis of the volume within the patient, and
further comprising deflecting the ultrasonic signal transversally with
respect to the longitudinal axis by a rotating reflective element.

16. The method of claim 1, wherein the generating the ultrasonic signal
comprises directing the ultrasonic signal transversally with respect to a
longitudinal axis of the volume within the patient by a rotating
transducer.

17. The method of claim 1, wherein the spot within the patient has a
diameter of less than 50 μm.

18. The method of claim 1 wherein the spot within the patient is located
at a depth of about 10 mm, or less from a surface of a tissue structure
within the patient.

19. The method of claim 1, wherein the generating the ultrasonic signal
comprises directing the ultrasonic signal at an angle from about
0.degree. to about 180.degree. relative to a longitudinal axis of the
volume within the patient.

20. The method of claim 19 further comprising: measuring a frequency
shift in the ultrasonic echo signal relative to the generated focused
ultrasonic signal; and determining a flow rate of a fluid in a lumen of
the volume within the patient using the measured frequency shift.

21. The method of claim 1 further comprising: displacing the ultrasonic
transducer along a longitudinal axis of the volume within the patient;
and providing a plurality of cross-sectional images of the volume within
the patient collected along the longitudinal axis.

22. The method of claim 21 wherein displacing the ultrasonic transducer
comprises at least one of the steps selected from the group consisting of
manually displacing the transducer and automatically displacing the
transducer.

23. The method of claim 21 wherein displacing the ultrasonic transducer
comprises at least one of the steps selected from the group consisting of
retracting the transducer and advancing the transducer.

24. A method for recanalization of a blood vessel, the method comprising:
positioning a flexible member within the blood vessel proximal to a
pre-selected area of interest; generating a focused ultrasonic signal
using a transducer, the ultrasonic signal forming a path within the blood
vessel; directing the ultrasonic signal on a spot within the blood
vessel; scanning the spot in a predetermined pattern about a volume
within the patient; receiving an ultrasonic echo in the transducer;
converting the ultrasonic echo into a voltage; selecting a frequency band
from the voltage; amplifying the voltage in the selected frequency band
with a processing circuit; and recanalizing a lumen in the blood vessel
based on the image.

25. The method of claim 24 wherein the pre-selected area of interest
comprises a stenosed segment of the blood vessel; and the predetermined
pattern comprises the stenosed segment of the blood vessel.

26. The method of claim 25 wherein the recanalizing the lumen in the
blood vessel based on the image comprises at least one of the steps in
the group consisting of: directing an ablation laser to a point of
interest; removing a portion of the stenosed segment of the blood vessel
with an abrasive surface; and delivering a drug to the point of interest.

27. The method of claim 24, further comprising classifying a plaque
within the wall of the blood vessel.

28. The method of claim 27 wherein the recanalizing the lumen in the
blood vessel is performed when the classifying a plaque indicates that
the plaque is a vulnerable plaque.

Description:

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application claims the benefit of the filing date of
provisional U.S. Patent Application No. 61/750,085 filed Jan. 8, 2013.
The entire disclosure of this provisional application is incorporated
herein by this reference.

BACKGROUND

[0002] 1.--Field of the Invention

[0003] The present disclosure relates generally to ultrasound imaging
inside the living body and, in particular, to a focused intravascular
ultrasound (IVUS) imaging catheter that produces high resolution
intravascular imaging using a polymer based transducer.

[0004] 2.--Description of Related Art

[0005] Intravascular ultrasound (IVUS) imaging is widely used in
interventional cardiology as a diagnostic tool for a diseased vessel,
such as an artery, within the human body to determine the need for
treatment, to guide the intervention, and/or to assess its effectiveness.
IVUS imaging uses ultrasound echoes to create an image of the vessel of
interest. Ultrasound waves pass easily through most tissues and blood,
but they are partially reflected from discontinuities arising from tissue
structures (such as the various layers of the vessel wall), red blood
cells, and other features of interest. The IVUS imaging system, which is
connected to the IVUS catheter by way of a patient interface module
(PIM), processes the received ultrasound echoes to produce a
cross-sectional image of the vessel where the catheter is placed.

[0006] Current IVUS solutions do not provide resolution capable of
differentiating structures without significant training in image
interpretation. Structures requiring clearer images might include plaque
burden, stent apposition, lipid pool identification, thrombus, and stent
endothelization. While Optical Coherence Tomography (OCT) devices offer
improved resolution, they require flushing to produce the image, and due
to limitations of light penetration they do not allow for visualization
of the vessel morphology beyond the surface of the vessel. While existing
IVUS catheters deliver useful diagnostic information, there is a need for
enhanced image quality to provide more valuable insight into the vessel
condition. For further improvement in image quality in rotational IVUS,
it is desirable to use a transducer with broader bandwidth and to
incorporate focusing into the transducer.

[0007] What is needed is a method for high resolution ultrasound imaging
to assess lesions, characterize vessels or to monitor other structures
within a patient's body.

SUMMARY

[0008] According to embodiments disclosed herein, a method for acoustic
tomography within a patient may include generating a focused ultrasonic
signal using a transducer, the ultrasonic signal forming a path within
the patient; directing the ultrasonic signal on a spot within the
patient; scanning the spot in a predetermined pattern about a volume
within the patient; receiving an ultrasonic echo in the transducer;
converting the ultrasonic echo into a voltage; selecting a frequency band
from the voltage; amplifying the voltage in the selected frequency band
with a processing circuit; and generating an image of the volume within
the patient structure utilizing the amplified voltage.

[0009] Further according to some embodiments, a method for recanalization
of a blood vessel may include positioning a flexible member within the
blood vessel proximal to a preselected area of interest; generating a
focused ultrasonic signal using a transducer, the ultrasonic signal
forming a path within the blood vessel; directing the ultrasonic signal
on a spot within the blood vessel; scanning the spot in a predetermined
pattern about a volume within the patient; receiving an ultrasonic echo
in the transducer; converting the ultrasonic echo into a voltage;
selecting a frequency band from the voltage; amplifying the voltage in
the selected frequency band with a processing circuit; and recanalizing a
lumen in the blood vessel based on the image.

[0010] These and other embodiments of the present invention will be
described in further detail below with reference to the following
drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1A is a schematic illustration of an intravascular ultrasound
(IVUS) imaging system, according to some embodiments.

[0012] FIG. 1B is a cross-sectional side view of a distal portion of a
catheter used in an IVUS imaging system, according to some embodiments.

[0013]FIG. 2A is a partial illustration of a focusing transducer,
according to some embodiments.

[0014]FIG. 2B is a partial illustration of a focusing transducer,
according to some embodiments.

[0015]FIG. 3 is a cross-sectional illustration of an IVUS catheter inside
a blood vessel, according to some embodiments.

[0016]FIG. 4 is a longitudinal illustration of an IVUS catheter inside a
blood vessel, according to some embodiments.

[0017]FIG. 5 is a longitudinal illustration of an IVUS catheter inside a
blood vessel, according to some embodiments.

[0018]FIG. 6 is a flow chart illustrating steps in a method for focused
acoustic computed tomography (FACT), according to some embodiments.

[0019]FIG. 7 is a flow chart illustrating steps in a method for
recanalization of a blood vessel using FACT, according to some
embodiments.

[0020] In the figures, elements having the same reference number have the
same or similar functions.

DETAILED DESCRIPTION

[0021] For the purposes of promoting an understanding of the principles of
the present disclosure, reference will now be made to the embodiments
illustrated in the drawings, and specific language will be used to
describe the same. It is nevertheless understood that no limitation to
the scope of the disclosure is intended. Any alterations and further
modifications to the described devices, systems, and methods, and any
further application of the principles of the present disclosure are fully
contemplated and included within the present disclosure as would normally
occur to one skilled in the art to which the disclosure relates. In
particular, it is fully contemplated that the features, components,
and/or steps described with respect to one embodiment may be combined
with the features, components, and/or steps described with respect to
other embodiments of the present disclosure. For the sake of brevity,
however, the numerous iterations of these combinations will not be
described separately.

[0022] In a typical rotational IVUS catheter, a single ultrasound
transducer element is located at the tip of a flexible driveshaft that
spins inside a plastic sheath inserted into the vessel of interest. The
transducer element is oriented such that the ultrasound beam propagates
generally perpendicular to the axis of the catheter. A fluid-filled
sheath protects the vessel tissue from the spinning transducer and
driveshaft while permitting ultrasound signals to freely propagate from
the transducer into the tissue and back. As the driveshaft rotates
(typically at 30 revolutions per second), the transducer is periodically
excited with a high voltage pulse to emit a short burst of ultrasound.
The same transducer then listens for the returning echoes reflected from
various tissue structures, and the IVUS imaging system assembles a two
dimensional display of the vessel cross-section from a sequence of
several hundred of these pulse/acquisition cycles occurring during a
single revolution of the transducer. In some embodiments, software may be
used to provide reconstruction three-dimensional (3D) images of tissue
structures by storing two-dimensional data collected from a rotational
IVUS catheter.

[0023] In the rotational IVUS catheter, the ultrasound transducer is
typically a piezoelectric element with low electrical impedance capable
of directly driving an electrical cable connecting the transducer to the
imaging system hardware. In this case, a four wire (or quad cable) can be
used to carry the transmit pulse from the system to the transducer and to
carry the received echo signals from the transducer back to the imaging
system by way of a patient interface module ("PIM") where the echo
signals can be assembled into an image. To transport the electrical
signal across a rotating mechanical junction some embodiments include an
electromechanical interface where the electrical signal traverses the
rotating junction. In some embodiments of a rotational IVUS imaging
system a rotary transformer, slip rings, and rotary capacitors, may be
used to create an electrical interface between the PIM and the catheter.

[0024] As described in more detail below, ultrasound transducers may be
formed to emit a focused beam. Utilizing a focused beam and/or alternate
piezoelectric materials allow Focused Acoustic Computed Tomography (FACT)
technologies to provide sub 50 μm resolution without compromising
depth or penetration. Thereby generating an image which is useful for
defining vessel morphology, beyond surface characteristics. Reference
will now be made to a particular embodiment of the concepts incorporated
into an intravascular ultrasound system. However, the illustrated
embodiments and uses thereof are provided as examples only, without
limitation on other systems and uses, such as but without limitation,
imaging within any vessel, artery, vein, lumen, passage, tissue or organ
within the body. Embodiments of focused acoustic computed tomography
methods as disclosed herein may also be used for renal denervation
applications.

[0025] FIG. 1A is a schematic illustration of an intravascular ultrasound
(IVUS) imaging system 100, according to some embodiments. IVUS imaging
system 100 includes an IVUS catheter 102 coupled by a patient interface
module (PIM) 104 to an IVUS control system 106. In some embodiments, a
bedside utility box (BUB) or a Bedside Interface Box (BIB) may be used as
an interface module. Control system 106 is coupled to a monitor 108 that
displays an IVUS image (such as an image generated by the IVUS system
100).

[0026] In some embodiments, IVUS catheter 102 is a rotational IVUS
catheter, which may be similar to a Revolution® Rotational IVUS
Imaging Catheter available from Volcano Corporation and/or rotational
IVUS catheters disclosed in U.S. Pat. No. 5,243,988 and U.S. Pat. No.
5,546,948, both of which are incorporated herein by reference in their
entirety, for all purposes. Catheter 102 includes an elongated, flexible
catheter sheath 110 (having a proximal end portion 114 and a distal end
portion 116) shaped and configured for insertion into a lumen of a blood
vessel (not shown). A longitudinal axis LA of the catheter 102 extends
between proximal end portion 114 and distal end portion 116. Catheter 102
is flexible such that it can adapt to the curvature of the blood vessel
during use. In that regard, the curved configuration illustrated in FIG.
1A is for exemplary purposes and in no way limits the manner in which the
catheter 102 may curve in other embodiments. Generally, catheter 102 may
be configured to take on any desired straight or arcuate profile when in
use.

[0027] In some embodiments a rotating imaging core 112 extends within
sheath 110. Accordingly, in some embodiments imaging core 112 may be
rotated while sheath 110 remains stationary. Imaging core 112 has a
proximal end portion 118 disposed within the proximal end portion 114 of
sheath 110 and a distal end portion 120 disposed within the distal end
portion 116 of sheath 110. Distal end portion 116 of sheath 110 and the
distal end portion 120 of imaging core 112 are inserted into the vessel
of interest during operation of IVUS imaging system 100. The usable
length of catheter 102 (for example, the portion that can be inserted
into a patient, specifically the vessel of interest) can be any suitable
length and can be varied depending upon the application. The proximal end
portion 114 of sheath 110 and the proximal end portion 118 of imaging
core 112 are connected to PIM 104. Proximal end portions 114, 118 are
fitted with a catheter hub 124 that is removably connected to PIM 104.
Catheter hub 124 facilitates and supports a rotational interface that
provides electrical and mechanical coupling between catheter 102 and PIM
104.

[0028] Distal end portion 120 of imaging core 112 includes a transducer
assembly 122. Transducer assembly 122 is configured to be rotated (either
by use of a motor or other rotary device) to obtain images of the vessel.
Transducer assembly 122 can be of any suitable type for visualizing a
vessel and, in particular, a stenosis in a vessel. In the depicted
embodiment, transducer assembly 122 includes a piezoelectric
micro-machined ultrasonic transducer ("PMUT") and associated circuitry,
such as an application-specific integrated circuit (ASIC). An exemplary
PMUT used in IVUS catheters may include a polymer piezoelectric membrane,
such as that disclosed in U.S. Pat. No. 6,641,540, and co-pending
applications entitled "Preparation and Application of a Piezoelectric
Film for an Ultrasound Transducer," Attorney Docket No. 44755.1062,
"Focused Rotational IVUS Transducer Using Single Crystal Composite
Material," Attorney Docket No. 44755.931, and "'Transducer Mounting
Arrangements and Associated Methods for Rotational Intravascular
Ultrasound (IVUS) Devices," Attorney Docket No. 44755.960, each hereby
incorporated by reference in its entirety. The PMUT may provide greater
than about 70% bandwidth, or about 75% bandwidth for optimum resolution
in a radial direction, and a spherically-focused aperture for optimum
azimuthal and elevation resolution. In some embodiments, a bandwidth of
about 75% may be sufficient to obtain high quality images. That is, in
some embodiments a transducer assembly fabricated using a PMUT material
according to embodiments as disclosed herein may have a response
bandwidth that is more than 100% of the center frequency of the response
band. For example, if the response band of transducer assembly 122 is 20
MHz, a response bandwidth may be about 20 MHz or more. Thus, the response
bandwidth of such transducer assembly may include frequencies from about
10 MHz to about 30 MHz.

[0029] Transducer assembly 122 may also include a housing having the PMUT
and associated circuitry disposed therein. In some embodiments the
housing has an opening that ultrasound signals generated by the PMUT
transducer travel through. Alternatively, transducer assembly 122 may
include a capacitive micro-machined ultrasonic transducer ("CMUT").
Accordingly, some embodiments may use a flat transducer assembly 122 with
an acoustic lens positioned adjacent to the transducer assembly, for beam
focusing. In yet another alternative embodiment, transducer assembly 122
could include an ultrasound transducer array (for example, arrays having
16, 32, 64, or 128 elements are utilized in some embodiments) utilizing
focus transducer assemblies.

[0030] The rotation of imaging core 112 within sheath 110 is controlled by
PIM 104. For example, PIM 104 provides user interface controls that can
be manipulated by a user. In some embodiments PIM 104 may receive and
analyze information received through imaging core 112. It will be
appreciated that any suitable functionality, controls, information
processing and analysis, and display can be incorporated into PIM 104.
Thus, PIM 104 may include a processor circuit 154 and a memory circuit
155 to execute operations on catheter 102 and receive, process, and store
data from catheter 102. In some embodiments PIM 104 receives data from
ultrasound signals (echoes) detected by imaging core 112 and forwards the
received echo data to control system 106. Control system 106 may include
a processor circuit 156 and a memory circuit 157 to execute operations on
catheter 102 and receive, process, and store data from catheter 102. In
some embodiments PIM 104 performs preliminary processing of the echo data
prior to transmitting the echo data to control system 106. PIM 104 may
perform amplification, filtering, and/or aggregating of the echo data,
using processor circuit 154 and memory circuit 155. PIM 104 can also
supply high- and low-voltage DC power to support operation of catheter
102 including the circuitry within transducer assembly 122.

[0031] In some embodiments, wires associated with IVUS imaging system 100
extend from control system 106 to PIM 104. Thus, signals from control
system 106 can be communicated to PIM 104 and/or vice versa. In some
embodiments, control system 106 communicates wirelessly with PIM 104.
Further according to some embodiments, catheter 102 may communicate
wirelessly with PIM 104. Similarly, it is understood that, in some
embodiments, wires associated with the IVUS imaging system 100 extend
from control system 106 to monitor 108 such that signals from control
system 106 can be communicated to monitor 108 and/or vice versa. In some
embodiments, control system 106 communicates wirelessly with monitor 108.

[0032] The piezoelectric micro-machined ultrasound transducer (PMUT)
fabricated using a polymer piezoelectric material, such as disclosed in
U.S. Pat. No. 6,641,540 that is hereby incorporated by reference in its
entirety, offers greater than about 70% bandwidth, or about 75% bandwidth
for optimum resolution in the radial direction, and a spherically-focused
aperture for optimum azimuthal and elevation resolution. The electrical
impedance of the transducer may be reduced to efficiently drive the
electrical cable coupling the transducer to the IVUS imaging system by
way of the PIM.

[0033] FIG. 1A illustrates a 3-dimensional (3D) Cartesian coordinate
system XYZ oriented such that the Z-axis is aligned with the LA. In
further descriptions of embodiments disclosed herein, a reference to a
Cartesian plane or coordinate may be made in relation to FIG. 1A. One of
ordinary skill will recognize that the particular choice of coordinate
axes in FIG. 1A is not limiting of embodiments as disclosed herein. The
choice of coordinate axes is done for illustration purposes only.

[0034] FIG. 1B is a cross-sectional side view of a distal portion of a
catheter used in an IVUS imaging system, according to some embodiments.
In particular, FIG. 1B shows an expanded view of aspects of the distal
portion of imaging core 112. In this exemplary embodiment, imaging core
112 is terminated at its distal tip by a housing 126 having a rounded
nose and a cutout 128 for the ultrasound beam 150 to emerge from the
housing. In some embodiments, a flexible driveshaft 132 of imaging core
112 is composed of two or more layers of counter wound stainless steel
wires, welded, or otherwise secured to housing 126 such that rotation of
the flexible driveshaft also imparts rotation to housing 126. In the
illustrated embodiment, a PMUT MEMS transducer layer 121 includes a
spherically focused portion facing cutout 128. In some embodiments,
transducer assembly 122 may include application-specific integrated
circuit (ASIC) 144 within distal portion 120 of imaging core 112. ASIC
144 is electrically coupled to transducer layer 221 through two or more
connections. In that regard, in some embodiments of the present
disclosure ASIC 144 may include an amplifier, a transmitter, and a
protection circuit associated with PMUT MEMS layer 121. In some
embodiments, ASIC 144 is flip-chip mounted to a substrate of the PMUT
MEMS layer 121 using anisotropic conductive adhesive or suitable
alternative chip-to-chip bonding method. When assembled together PMUT
MEMS layer 121 and ASIC 144 form an ASIC/MEMS hybrid transducer assembly
122 mounted within housing 126. An electrical cable 134 with optional
shield 136 may be attached to transducer assembly 122 with solder 140.
Electrical cable 134 may extend through an inner lumen of the flexible
driveshaft 132 to proximal end 118 of imaging core 112. In proximal end
118, cable 134 is terminated to an electrical connector portion of a
rotational interface coupling catheter 102 to PIM 104 (cf. FIG. 1A). In
the illustrated embodiment, transducer assembly 122 is secured in place
relative to the housing 126 by an epoxy 148 or other bonding agent. Epoxy
148 may serve as an acoustic backing material to absorb acoustic
reverberations propagating within housing 126 and as a strain relief for
the electrical cable 134 where it is soldered to transducer assembly 122.

[0035]FIG. 2A is a partial illustration of a focusing transducer 122A,
according to some embodiments. Transducer 122A includes a polymeric layer
221 having a first adjacent conductive layer 222a and a second adjacent
conductive layer 222b. Polymeric layer 221 includes a piezo-electric
polymer material made into a concave shape as depicted in FIG. 2A. In
some embodiments, the polymer used in polymeric layer 221 may be a
ferroelectric polymer such as polyvinylidene fluoride (PVDF). Further
according to some embodiments, polymeric layer 221 may include
PVDF-co-trifluoroethylene (PVDF-TrFE) as a piezo-electric material. A
voltage 230 (V) is applied between conductive layers 222a and 222b in
order to generate focused ultrasound beam 250A. Likewise, in some
embodiments an incident ultra-sound beam 250A may impinge on polymeric
layer 221 and produce a deformation leading to a voltage difference V 230
between conductive layers 222a and 222b.

[0036] In some embodiments, the concavity of transducer 122A may be a
section of a sphere. In some embodiments, the concavity of transducer
122A is directed radially outward, in a plane perpendicular to the LA
(i.e., XY-plane in FIG. 2A). Accordingly, in rotational IVUS embodiments,
transducer 122A rotates about the LA, thus sweeping focused beam 250A
radially in the XY plane. In some embodiments, while transducer 122A may
include a planar polymeric layer, an acoustic `lens` may be placed
adjacent to transducer 122A. Thus, focused acoustic beam 250A may be
generated by acoustic wave refraction. Still further, the material
forming sheath 110 may have an acoustic impedance, thereby focusing the
acoustic wave propagating through sheath 110.

[0037]FIG. 2B is a partial illustration of a focusing transducer 122B,
according to some embodiments. Transducer 122B may have a polymeric layer
221 with a concavity oriented along the LA. Focusing transducer 122B
includes a rotating reflective element 225 to direct focusing ultrasonic
beam 250B radially out of axial direction LA. According to some
embodiments, focused ultrasonic beam 250B may be generated substantially
along the blood vessel, or the LA axis. Then, beam 250B may be deflected
radially outward by rotating reflective element 225, as shown in FIG. 2B.

[0038] Still further, the output of the transducer or a reflecting element
may be oriented to generally align with the longitudinal axis LA. These
devices may be swept through an arc to generate forward looking images.

[0039] Focused ultrasonic beams 250A, 250B have a focal distance 210 (f)
converging into a focal waist 220 (ω). Accordingly, the focal waist
has a diameter that may be less than 50 μm. Focal distance 210 is
determined from the curvature of the surface formed by transducers 122A,
122B, and the refractive index of the propagation medium of focused
acoustic beam 250. Typically, the propagation medium is blood, plasma, a
saline solution, or some other bodily fluid. In some embodiments, focal
distance `f` may be as long as 10 mm, or more. Thus, the tissue
penetration depth of focused ultrasonic beams 250A, 250B may be 5 mm, 10
mm, or more.

[0040] Focal distance 210 and focal waist 220 may also be determined by
the curvature of the aperture. In some embodiments focused acoustic beam
250A, B may include a plurality of acoustic frequencies in a frequency
bandwidth. The frequency bandwidth may be determined by the polymer
material and the shape of polymeric layer 221. The structure of the
transducer assembly including backing, electrodes, and matching layers
may determine the acoustic frequency bandwidth of transducers 122A, 122B.
The viscoelastic properties of the polymer material may also determine
the acoustic frequency bandwidth of transducers 122A, 122B.

[0041] Accordingly, some embodiments have a polymeric layer 221 such that
an ultrasonic signal produced by transducers 122A, 122B includes a
frequency bandwidth from about 5 to about 135 Mega Hertz (MHz, 1
MHz=106 Hz). Embodiments of catheters 102 including a transducer
such as transducers 122A, 122B allow for a better image resolution since
ultrasonic beams 250A, 250B are focused.

[0042] Further according to some embodiments, the material and shape of
distal portion 116 of sheath 110 may be selected to match the acoustic
impedance of the materials in transducer 122 and the target structure
(e.g., blood vessel wall). Impedance matching of the acoustic signal
across all elements in the distal portion of catheter 102 is desirable to
enhance the response of transducer 122 to the acoustic echo coming from
the blood vessel wall. Embodiments of materials and shapes of distal
portion 116 of sheath 110 to match the acoustic impedance in transducer
122 may be as disclosed in co-pending U.S. Patent Application entitled
"Intravascular Ultrasound Catheter for Minimizing Image Distortion,"
Attorney Ref. No. 44755.938, hereby incorporated by reference in its
entirety, for all purposes. Impedance matching layers can also be used to
modify impedance match

[0043]FIG. 3 is a cross-sectional illustration of an IVUS catheter 102
inside a blood vessel 300, according to some embodiments. Blood vessel
300 includes a lumen 310, typically filled with a blood flow. FIG. 3 also
shows a stenosed segment in blood vessel 300. The stenosis may include a
plaque formed by a fibrous cap 320 adjacent to a necrotic core 330 formed
in an interior side of a layer adventitia 340. Accordingly, methods for
using an IVUS catheter in FACT enable the classification of a plaque in a
stenosed segment of a blood vessel, such as illustrated in FIG. 3. For
example, a plaque may be classified as `vulnerable` to rupture, based on
the size, configuration, and nature of its components. A component of a
plaque within a vessel may be fibrous cap 320, and necrotic core 330. In
some configurations a component of a plaque within a vessel may include
fat cell tissue and macrophage cells. The nature of a component of a
plaque within a vessel may include substances such as elastin, collagen
and cholesterol. The viscoelastic properties of the substances and the
configuration of the different components included in a plaque within a
vessel provide a differentiated acoustic response. Thus, interaction with
a focused ultrasound beam 350 may produce an image that clearly
differentiates the components of the plaque, their nature, and their
configuration (size and shape).

[0044]FIG. 3 depicts a focused ultrasonic beam 350 directed axially from
catheter 102 toward the wall of blood vessel 300. Ultrasonic beam 350 is
generated by transducer 122 inside distal end portion 120 of imaging core
112, and passes through distal end portion 116 of sheath 110 and blood
plasma or a saline solution in lumen 310. According to some embodiments,
focused ultrasonic beam 350 may be rotating about the LA of catheter 102,
in a trajectory projected as a circle in the XY plane in FIG. 3.

[0045] In some embodiments, focused ultrasonic beam 350 may be reflected
from the wall of blood vessel 300 towards transducer 122 in distal end
portion 120 of imaging core 112. Thus, a reflected ultrasound signal may
be recorded by PIM 104, providing information about the tissue in the
wall of blood vessel 300. The reflected ultrasound signal may be the echo
of a focused ultrasound signal projected onto the wall of blood vessel
300 from transducer 122 in the distal end 120 of imaging core 112. Beam
350 sweeps about the LA forming an arc, scanning a volume of the
patient's tissue.

[0046]FIG. 4 is a longitudinal illustration of an IVUS catheter 102
inside a blood vessel 300, according to some embodiments. Focused
ultrasound beam 450 is directed radially onto blood vessel wall 340 in a
direction forming an azimuthal angle 455 (θ) with axis LA (along
the Z-axis). Accordingly, angle 455 may have any value between zero
(0°) and ninety (90°) degrees. In some embodiments, angle
455 may be larger than 90°, and close to 180°. FIG. 4 shows
a blood 410 flowing in a direction substantially away from distal end 116
of catheter 102. In some embodiments, blood 410 may be flowing in a
direction substantially away from a proximal end of catheter 102 (i.e.,
opposite of what is shown in FIG. 4). In general, blood flow is
substantially parallel to the LA (Z-axis in FIG. 4), either along the +Z
direction, or along the -Z direction.

[0047] In some embodiments as illustrated in FIG. 4, angle 455 may be
90° so that a transverse scan is obtained as focused ultrasound
beam 450 is rotated about the LA. Thus, a transverse plane scan
substantially parallel to the XY-plane may be obtained.

[0048] In some embodiments angle 455 is not perpendicular (90°) to
the LA. For example, in some embodiments angle 455 may be less than
90° in a forward looking IVUS catheter. In configurations where
angle 455 is not perpendicular, a component of the blood flow velocity
along the direction of focused ultrasound beam 450 may be different from
zero (0). When this is the case, an acoustic echo received from blood
vessel wall 340 may be slightly shifted in frequency, by virtue of the
Doppler effect. The frequency shift of the ultrasonic echo is related to
the component of the flow velocity along the direction of focused
ultrasound beam 450. For example, a frequency shift of the ultrasonic
echo signal may be directly proportional to the magnitude of the
component of the flow velocity along the direction of focused ultrasound
beam 450. Thus, using knowledge of angle 455 and measuring the frequency
shift of the echo signal, a blood flow speed may be obtained.

[0049]FIG. 5 is a longitudinal illustration of an IVUS catheter 102
inside a blood vessel 300, according to some embodiments. According to
FIG. 5, IVUS catheter 102 axially advances along blood vessel 300. Blood
vessel 300 may include a stenosed segment 500, having a plaque. The
plaque within vessel 300 may include fibrous cap 320 on top of necrotic
tissue 330. Blood vessel 300 also includes muscle cell tissue 340, as
described in detail above (cf. FIGS. 3-4).

[0050] Focused ultrasound beam 350 forms a predetermined pattern 550 as it
rotates around LA and catheter 102 is displaced in the +Z direction. For
example, in some embodiments consistent with the present disclosure a
predetermined pattern 550 may be a helicoid trajectory. In some
embodiments, transducer 122 is retracted from, or pulled back through
blood vessel 300. That is, distal end 116 is displaced in the -Z
direction, according to some embodiments. Advancing and retracting
transducer 122 along vessel 300 may be accomplished manually or with an
automated system. Without limiting embodiments of the present disclosure,
a retracted displacement (along the -Z direction) of transducer 122 will
be assumed hereinafter.

[0051] As transducer 122 is retracted through the blood vessel 300,
ultrasound echo signals collected along pattern 550 may be used to create
an image of the blood vessel wall. The image of the blood vessel wall may
be a 3D image including a plurality of cross sections of the blood vessel
wall. The cross sections of the blood vessel wall may be substantially
aligned with planes XY perpendicular to the Z-axis (i.e., the LA
direction), along different points on the Z-axis. The image generated
from pattern 550 may be processed by PIM 104 and control system 106 using
an image characterization application, or code. The image
characterization application may render a characterized tissue component
map. In some embodiments, the image characterization application performs
a spectral analysis of ultrasound echo information for a vessel
cross-section. Thus, different plaque components may be determined and
distinguished in the characterized tissue component map. For example, the
characterization application may use a classification criterion including
a rule based upon a location of a confluence of necrotic core 330 within
the vessel cross-section in relation to a border between the lumen 310
and a plaque. A classification criterion may include a rule, based upon a
location, in relation to a lumen-plaque border, of confluent necrotic
core within the vessel cross-section; and rendering, in response to the
classification, a plaque classification associated with the vessel
cross-section. For example, a classification criterion may use the
thickness of fibrous cap 320 to determine the vulnerability of a plaque,
and the likelihood of plaque rupture and thrombosis.

[0052]FIG. 6 is a flow chart illustrating steps in a method 600 for focus
acoustic computed tomography (FACT), according to some embodiments.
According to some embodiments, method 600 may be performed by control
system 106 using processor circuit 156 and memory circuit 157 and/or PIM
104 using processor circuit 154 and memory circuit 155, based on scan
data provided by transducer assembly 122 (cf. FIG. 1). Accordingly, some
steps in method 600 may be performed by control system 106 and some steps
in method 600 may be performed by PIM 104. A reconstructed image plane in
method 600 may be provided to a user in display 108.

[0053] Step 610 includes generating an ultrasonic signal. In some
embodiments, the ultrasonic signal may be generated from a transducer
assembly (e.g., transducer assembly 122, cf. FIG. 1) in the form of an
ultrasound acoustic beam following a path. The ultrasound acoustic beam
path may be a focused path (e.g., focused acoustic beam 350, cf. FIG. 3)
to produce a focal region of high acoustic intensity in a target portion
of a tissue. Step 610 may include providing a voltage pulse to a
transducer element (e.g., V 230, cf. FIGS. 2A, B). Accordingly, voltage
pulse V 230 may be provided by PIM 104 to transducers 122A, B for a
pre-selected period of time in step 610. Furthermore, voltage pulse V 230
may be provided in a series of pulses produced at a preselected
frequency, in step 610.

[0054] Step 620 includes scanning the ultrasonic signal in a predetermined
pattern about the interior wall of a structure. In some embodiments, step
620 may include sweeping the ultrasonic signal continuously in a
predetermined pattern about the interior vessel wall. According to some
embodiments, the sweeping is accomplished by rotating the transducer or a
reflective surface which deflects the signal from the transducer within a
catheter. The catheter may remain substantially stationary while the
transducer is rotated and the acoustic signal is swept around the LA of
the catheter. In some embodiments, the transducer is moved
longitudinally, as it rotates about the LA to create a helical pattern
(e.g., pattern 550, FIG. 5). In some embodiments, step 620 may be
performed manually by an operator, or automatically through a motor
included in PIM 104.

[0055] Step 630 includes receiving an ultrasonic echo from the interior
wall of the structure. According to some embodiments, step 630 may be
performed using a transducer, such as transducers 122A, 122B described in
detail above, in relation to FIGS. 2A and 2B. Thus, upon receiving the
ultrasonic echo from the interior wall of the structure, a deformation
induced in a piezo-electric material in the transducer may result in a
voltage signal. The transducer may further be configured to couple the
voltage signal out of a vessel region into a processor circuit, such as
processor circuit 154 in PIM 104.

[0056] In some embodiments, step 630 may be performed during a period of
time between two voltage pulses from step 610. Thus, according to some
embodiments in step 610 a voltage signal travels from processor circuit
154 in PIM 104 into transducer assembly 122 along catheter 102 (cf. FIG.
1). Further according to some embodiments, in step 630 a voltage signal
travels from transducer assembly 122 to processor circuit 154 in PIM 104.
In some embodiments, step 630 may include selecting a frequency band from
the voltage received in the PIM. Accordingly, the high-bandwidth of
transducer 122 may enable the selection of different frequency bands
within the response band of the transducer. Selecting a specific
frequency band enables PIM 104 to reconstruct images from selected
portions of the tissue structure. For example, the focal distance of a
focused acoustic beam may be selected by selecting a frequency band in
the voltage received from the transducer in step 630. Thus, the
penetration depth of the acoustic echo signal may be selected in PIM 104
by selecting the frequency band of receive amplifier 114 in step 630.

[0057] Step 640 includes amplifying the ultrasonic echo in a processor
circuit. The processor circuit in step 640 may be as processor circuit
154 in PIM 104 (cf. FIG. 1). According to some embodiments, the processor
circuit in step 640 may include an ASIC provided adjacent to the
transducer, in a distal portion of the catheter (e.g., ASIC 144, cf. FIG.
1B). Step 650 includes producing an image from the ultrasonic echo.
According to some embodiments, step 650 may be performed partially by PIM
104. In some embodiments, step 650 may be performed partially by control
system 106. According to some embodiments, step 650 includes producing a
2-dimensional image (2D-image) of a cross section of the blood vessel
wall. The cross-section may be substantially parallel to an XY-plane
perpendicular to a LA oriented along the blood vessel. In some
embodiments, step 650 includes producing a 3-dimensional image (3D-image)
of the blood vessel wall from a plurality of 2D-images. According to
embodiments as described herein, step 650 may include forming an image
with axial resolution better than 50 μm.

[0058] In some embodiments, step 650 may include complementing the image
obtained from the IVUS catheter with an image obtained using an optical
beam scanning technique, such as optical coherence tomography (OCT). For
example, using an OCT system a high resolution image of a deep tissue
structure may be obtained. Such a deep tissue image may be complemented
with an IVUS image of tissue portions close to the catheter, including
blood stream in a blood vessel.

[0059] Step 660 includes classifying the image from the ultrasonic echo.
Step 660 may be performed by processor circuit 156 and memory circuit 157
in control system 106. In some embodiments, step 660 is performed by
processor circuit 156 executing commands, retrieving and storing data,
the commands and the data being stored in memory circuit 157. For
example, in some embodiments the commands executed by processor circuit
156 in control system 106 may be included in an image characterization
application stored in memory circuit 157. The image characterization
application may be as described in detail above, in relation to FIG. 5.
Image characterization applications as used in step 660 may be as
disclosed in U.S. Pat. No. 7,627,156 entitled "Automated lesion analysis
based upon automatic plaque characterization according to a
classification criterion," U.S. Pat. No. 7,175,597 entitled "Non-Invasive
Tissue Characterization System and Method," and U.S. Pat. No. 6,200,268
entitled "Vascular Plaque Characterization," each incorporated herein by
reference in its entirety, for all purposes.

[0060]FIG. 7 is a flow chart illustrating steps in a method 700 for
recanalization of a blood vessel using FACT, according to some
embodiments. According to some embodiments, method 700 may be performed
partially by system 100 and an external operator handling a
recanalization tool. Steps performed by system 100 may be partially
executed by control system 106 using processor circuit 156 and memory
circuit 157. In some embodiments, steps performed by system 100 may be
partially executed by PIM 104 using processor circuit 154 and memory
circuit 155 based on scan data provided by transducer assembly 122 (cf.
FIG. 1). A reconstructed image plane in method 700 may be provided to the
external operator in display 108. Thus, the external operator makes a
decision of whether or not to excise a portion of the stenosed segment of
the blood vessel using a recanalization tool based on the reconstructed
image plane on display 108. According to some embodiments the
recanalization tool may be a physical instrument having a sharp end on an
abrasive surface. In some embodiments, the recanalization tool may be a
laser beam susceptible of being directed to a point in the blood vessel
and ablate a tissue portion.

[0061] Step 710 includes positioning a flexible member within a blood
vessel. A flexible member in step 710 may be as catheter 102, including a
rotating imaging core 112 and a sheath 110. Step 710 may further include
positioning catheter 102 with its LA substantially aligned with the blood
vessel, inside the lumen portion of the blood vessel. Step 720 includes
generating an ultrasonic signal. Step 720 may be as step 610, described
in detail above in relation to method 600. Step 730 includes scanning the
ultrasonic signal in a predetermined pattern about the interior wall of a
structure including a region of stenosis. In some embodiments, the
predetermined pattern may be as pattern 550 described in detail above
(cf. FIG. 5). The region of stenosis may also be as stenosed segment 500,
described in detail above (cf. FIG. 5).

[0062] Step 740 includes receiving an ultrasonic echo from the interior
wall of the structure. Accordingly, step 740 may be as step 630 described
in detail above, in relation to method 600. Step 750 includes amplifying
the ultrasonic echo in a processor circuit. Step 750 may be as step 640
described in detail above, in relation to method 600. Step 760 includes
producing an image from the region of stenosis, and may be as step 650
described in detail above in relation to method 600. Step 770 includes
classifying the image from the region of stenosis, and may be as step 660
described in detail above in relation to method 600.

[0063] Step 780 includes querying whether or not the region of stenosis
needs a recanalization procedure. A decision in step 780 may be made
according to the detected vulnerability of a plaque that may be present
in the region of stenosis. For example, when an image from the region of
stenosis is classified as `vulnerable plaque` in step 770, a
recanalization may be recommended in step 780. When an image from the
region of stenosis is classified in a category other than `vulnerable
plaque,` method 700 may be repeated from step 710. Thus, catheter 102 may
be re-positioned at a different point along the blood vessel.

[0064] Step 790 includes recanalizing the region of stenosis. According to
some embodiments, step 790 may be performed prior to steps 720 through
760. In some embodiments, step 790 may be performed after steps 720
through 760. Further according to some embodiments, step 790 may be
performed at any point during execution of any one of steps 720 through
760. In some embodiments step 790 may include providing heat to a target
region (e.g., stenosed segment 500, cf. FIG. 5) to mitigate the stenosis.
In some embodiments, step 790 may include ablation, use of an abrasive
surface, drug delivery, stenting, and drilling, to remove the stenosis.
For example, step 790 may include lightly rubbing an abrasive surface
against the tissue to remove a stenosed segment.

[0065] Embodiments of the invention described above are exemplary only.
One skilled in the art may recognize various alternative embodiments from
those specifically disclosed. Those alternative embodiments are also
intended to be within the scope of this disclosure. As such, the
invention is limited only by the following claims.

Patent applications by Cheryl Rice, San Diego, CA US

Patent applications by Volcano Corporation

Patent applications in class Anatomic image produced by reflective scanning

Patent applications in all subclasses Anatomic image produced by reflective scanning